It is well-known that the use of multiple antennas at the transmitter and/or the receiver can significantly boost the performance of a wireless system. Such antenna configurations have the potential of both improving data rates and increasing coverage.
Precoding is a multi-antenna technique for improving the performance of a multi-antenna system by transforming the information carrying transmit vector so that it better fits the channel conditions. Thus, precoding is a kind of beamforming that supports multi-layer transmission in Multiple-Input Multiple-Output (MIMO) radio systems and has been included in the standardization of Long Term Evolution (LTE).
In precoding, each of the multiple streams are emitted from each of the transmit antennas at the base station with independent and appropriate weighting per antenna element such that the throughput is maximized between the base station and the user equipment (UE). The benefits of the weighting are to increase the signal gain and/or to reduce interference from other users of the communication system. The precoding weights are calculated at the UE and then the UE informs the base station which precoding weights that should be used. Usually, only a limited number of predefined precoding weights are used, called a codebook. The codebook is known both at the base station and the UE, so when the UE informs the base station which precoding weights that should be used, the UE only needs to send a number corresponding to the place that the precoding weights have in the codebook. This number is usually called Precoding Matrix Indicator (PMI).
The more antennas that are used at the base station, the more precoding weights are needed in the codebook.
Precoding can be frequency selective or wideband. In frequency selective precoding different PMIs can be chosen to be applied in different frequency subbands. With wideband precoding the same PMI is used over the entire frequency band.
In order to meet future demands of higher bit rates a wider system bandwidth will be necessary. In LTE-Advanced, wider bandwidth will be achieved by aggregating component carriers either contiguously or non-contiguously. For contiguous carrier aggregation, consecutive spectrum is aggregated, while discontinuous bands are accumulated for non-contiguous carrier aggregation.
If the relative bandwidth is large or aggregated spectrum is used, a beam corresponding to a certain PMI will point in different directions for different frequencies within the band. Since the same PMI must be used over the entire frequency band within wideband precoding, this will lead to that a beam that points in a direction to the UE for one frequency will point away from the UE for other frequencies. FIG. 1 is a diagram which illustrates radiation patterns of an example of prior art wideband precoding over two subbands, the first located at 700 MHz and the second located at 1000 MHz. The first beam 110 is the radiation pattern for the subband located at 700 MHz and the second beam 120 is the radiation pattern for the subband located at 1000 MHz. As can be seen in FIG. 1, the first radiation pattern points at the mobile node 100 but the second radiation pattern have almost a null in the direction of the mobile node 100 which will lead to reduced Signal-to-Noise Ratio (SNR) for the second subband. Particularly, in a line-of-sight scenario or in a channel with small angular spread this will lead to a gain drop at these frequencies and degraded performance.